1. Field of the Invention
The present invention relates to a relief printing plate precursor for laser engraving, and a method for producing a relief printing plate.
2. Description of the Related Art
As a method for forming a printing plate by forming concavities and convexities on a photosensitive resin layer laminated on the surface of a support, so-called “analogue plate making,” is well known, which is a method comprising exposing a relief forming layer formed using a photosensitive composition to ultraviolet light via an original image film, selectively curing image parts, and removing uncured parts with a developing liquid.
A relief printing plate is an anastatic printing plate having a relief layer with concavities and convexities, and the relief layer with concavities and convexities is obtained by patterning a relief forming layer containing a photosensitive composition containing, as a main component, for example, an elastomeric polymer such as a synthetic rubber, a resin such as a thermoplastic resin, or a mixture of a resin and a plasticizer, and forming concavities and convexities thereon. Among such relief printing plates, one having a flexible relief layer may be referred to as a flexo plate.
In recent years, research has been carried out on a method of performing the plate making of a relief forming layer by scanning exposure, without requiring an original image film as in the case of analogue plate making.
As a technique that does not require an original image film, a relief printing plate precursor provided with a laser-sensitive mask layer element capable of forming an image mask on a relief forming layer, has been proposed (see, for example, U.S. Pat. No. 5,798,202 and Japanese Patent Application Laid-Open (JP-A) No. 2002-3665). According to these methods of plate making of a precursor, since an image mask having the same function as the original image film is formed from a mask layer element by laser irradiation based on image data, these methods are referred to as “mask CTP methods,” and original image films are not required. However, the subsequent plate making treatment involves processes of exposure to ultraviolet light through an image mask, and developing and removing uncured parts, and there is a room for further improvement from the viewpoint of the necessity of development treatment.
As a plate making method that does not require a development process, a so-called “direct engraving CTP method” that includes directly engraving a relief forming layer with a laser to carry out plate making, has been proposed in many cases. The direct engraving CTP method is literally a method of forming concavities and convexities that serves as a relief by engraving with a laser, and unlike the relief formation using an original image film, there is an advantage that the relief shape can be freely controlled. Accordingly, in the case of forming images like outline characters, it is possible to engrave the image regions deeper than other regions, and in the case of forming fine halftone dot images, it is possible to carry out shouldered engraving in consideration of resistance to the printing pressure.
In regard to the plate materials that have been so far used in the direct engraving CTP method, many materials have been proposed as the binder that determines the characteristics of the plate materials, such as a material using a hydrophobic elastomer (rubber) (see, for example, U.S. Pat. No. 5,798,202, Japanese Patent Application Laid-Open (JP-A) No. 2002-3665, Japanese Patent No. 3438404, Japanese Patent Application Laid-Open (JP-A) No. 2004-262135 and Japanese Patent Application Laid-Open (JP-A) No. 2001-121833), a material using a hydrophilic polyvinyl alcohol derivative (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-2061), and a material using an elastomer having a crosslinked structure (see, for example, Japanese Patent No. 2846954).
There has been also proposed a method of producing a flexo printing plate precursor having improved film strength and printing durability by photocuring a photosensitive resin composition including a resin having a polymerizable unsaturated group (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-221655).
However, in the above-described techniques, since the sensitivity is low, when concavities and convexities are formed in the relief forming layer so as to have an engraving depth to form a relief that is resistant to the printing pressure, high energy is required. Further, since the laser engraving speed is low, the productivity is low as compared to the case of forming an image through a mask.
As discussed above, various techniques have been proposed in regard to a resin composition that can be suitably used in the relief forming layer of a relief printing plate precursor for laser engraving, but under the current circumstances, a resin composition that exhibits good film strength when formed into a film, and thus provides excellent printing durability of the printing plate formed when the resin composition is applied to the relief forming layer, and further exhibits high engraving sensitivity when subjected to laser engraving, has not yet been provided.